Rhodopsin and related pigments are being studied using resonance Raman scattering techniques. This technique is a sensitive and powerful probe of in situ chromophore structure and state of protonation. Besides bovine rhodopsin, experimental work on bacteriorhodopsin, artificial pigments (made with retinal analogs) and model chromophores are envisioned. Theoretical studies will be undertaken in order to facilitate the interpretation of the data in detailed and reliable terms and reliable molecular models. There are three overall aims: (1) to define the structure of the chromophore site; (2) to determine the primary photochemical event; and (3) to determine the structure and reaction pathways of the dark intermediates that follow the formation of the primary photoproducts of rhodopsin and bacteriorhodopsin. The chromophore site will be examined by studying the changes of Raman spectra from retinal analogs in situ compared to their solution spectra and coupling this to a theoretical program to fully understand the results. Also, a novel kinetic deuterium exchange technique is proposed which will allow measurements of chromophore accessibility to small diffusible probes and the effects on this accessibility of the rhodopsin and bacteriorhodopsin photocycle. The Raman spectra of the batho products and artificial batho pigments will be obtained in order to define the structure of this intermediate in rhodopsin and bacteriorhodopsin so as to be able to develop a reliable molecular model for the primary photochemical event. The Raman spectrum of hypsorhodopsin is to be measured to more fully understand its role in the primary event.